This invention relates to an instrument for measuring light emission, and particularly to an instrument for repeatedly measuring periodic light emission with high timing resolution when the light intensity, beam shape and spectral distribution fluctuate at ultrahigh speed.
When an organic molecular crystal is stimulated by light pulses, fluorescence with a lifetime of the order of 10.sup.-6 to 10.sup.-9 second occurs in this crystal. The characteristics of the crystal may be known determined by accurately measuring a profile of light intensity changes with time, and also by measuring the spectral distribution. If gas discharging and other fluctuating light emission may be measured with high precision, the performance of light emitting devices, the chemical and physical characteristics of materials, and their changes can be known.
An example of the conventional method for measuring light emission fluctuating at a high speed is described hereinafter with reference to a method for measuring the lifetime of fluorescence which may occur in an organic molecular crystal.
In the so-called sampling method, fluorescence occurs in a crystal to be observed when the crystal is stimulated by applying repetitive light pulses clocking at a rate ranging from several Hz to several MHz. The fluorescence induced by said stimulation is detected in a photomultiplier tube. The sampling pulse voltage generated in accordance with said stimulation is applied to the dinode of said photomultiplier tube or a mesh electrode (hereinafter referred to as a gate) across the electron beam path, and the sampling is carried out each time the crystal is stimulated. By totalling the results of the sampling operations, a fluorescent light intensity profile may be obtained corresponding the wavelengths of fluorescence.
In the so-called streaking camera method, a crystal is stimulated by emitting a single light pulse. The light of fluorescence induced by said stimulation is incident on the photoelectric layer of a streaking tube. When a deflection voltage synchronized with the stimulation light pulse is applied to the deflection plate of the streaking tube, an image characterized by an intensity proportional to the intensity of the light of fluorescence and also by a timing axis scanned by the deflection is displayed on the phosphor screen of the streaking tube. By analyzing the image, a fluorescent light intensity profile can be obtained.
In a third measuring method, a fluorescent light intensity profile is statistically obtained. According to the method, a crystal to be observed is stimulated by repetitive light pulses clocking at a rate ranging from several Hz to several MHz. The light intensity of fluorescence occurring in a crystal is decremented through a filter so as to obtain such a value that a single photon can be detected. The decremented fluorescent light intensity is detected in a photomultiplier tube. An integral circuit starts operation when the stimulation light pulse is emitted and it is stopped when an output pulse of the photomultiplier is emitted. (This type of integral circuit is called a time-to-voltage converter.) The number of cycles in a unit time is counted with time as a parameter by measuring time in terms of the integral circuit output voltage which can be measured with a multichannel crest voltmeter. The number of cycles in a unit time is plotted with the output voltage as a parameter, and a profile of the fluorescent light intensity proportional to the number of cycles counted with time which is represented in terms of the output voltage is thus obtained.
Said sampling method has been used often, but a photomultiplier tube can detect only a fluorescent light intensity change with time because it can detect only information related to light intensity. For the purpose of making an accurate sampling by this method, the photomultiplier tube must satisfactorily be operated during the sampling. Hence, the sampling pulse voltage must be kept unchanged during the sampling. However, it is not easy to generate a square wave voltage with an accurate narrow pulse width. This limits the timing resolution to 5.times.10.sup.-9 second according to the sampling method.
In the above secondly mentioned streaking camera method, the timing resolution can be improved by increasing the voltage sweeping rate by rapidly changing the deflection voltage of the streaking tube. However, if the deflection voltage rate is increased so as to improve the timing resolution, sweeping on the phosphor screen is accomplished in a short period of time and it reduces the required measuring time. In other words, according to the streaking camera method an improvement in the timing resolution and increase of the measuring time do not conform, and they are not satisfied at the same time.
In the third and statistical method, the measurement is carried out by detecting a single photon and a number of sampling operations are required for a complete measurement. A relatively long period of time is thus required for measurement. For instance, if the sampling is carried out at 30 Hz, nine hours are required to accomplish the sampling of 10.sup.6 times during the measurement.